Equipment Specs

Dredge Mining

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Mining Processes
An example of a modern suction dredge used in small-scale placer mining operations.
Dredge mining
, a type of alluvial mining, involves the use of a floating boat or barge with either a series of buckets to scoop gravel, or a suctioning apparatus to vacuum gravel from the bottom of a creek or river. The most common type of dredge used in gold placer mining in the early 1900s was a bucket-line dredge. Today, suction dredges or modern dredges that use suction systems to vacuum up gold-bearing material are used. The use of dredges in mining is also accompanied with the use of sluices or screens to separate the gold from gravel and debris.


[edit] History

Some of the earliest dredges used in mining, such as those in New Zealand, were called spoon dredges. The spoon dredge featured a leather or canvas bag tied to an iron loop attached to a pole. The spoon was dragged towards the barge with a winch. As it was dragged, it scooped material from the bottom of the river or creek. The contents were then lifted and dumped into a sluice box. A waterwheel-driven pump supplied the water necessary for washing and separating gravel from the gold inside the sluice box. Spoon dredges were limited in their ability to dig very deep or work a large volume of gravel.[1]

[edit] Development of Bucket-line Dredge

The bucket-line dredge was developed around 1868. It quickly mechanized alluvial gold mining in Alaska and the Yukon where it was widely used after the Klondike Gold Rush. Bucket-line dredges featured a series of buckets connected to a long chain. The first bucket-line dredges had wheels powered by river currents. Steam-powered dredges and eventually electric-powered dredges appeared before the turn of the century.

A Chinese man by the name of Charles Sew Hoy of Dunedin from New Zealand had a steam-powered bucket-dredge built with a string of buckets hoist from a ladder that could be dropped down to the level of the riverbed and onto river flats on the shore. His design was thought to be the prototype for the New Zealand type of dredge that could remove gold from riverbeds, artificial ponds, and river flats. New Zealand’s gold dredges became renowned and were being imported overseas for use by 1890.[2]

An example of the mammoth sized bucket ladder gold dredges used in the Klondike and Alaska.

[edit] Mining Dredges in the Yukon and Alaska

The first a gold dredge was used in the Yukon was in 1899. Soon about two-dozen dredges were working the Yukon region. Dredges were used in a similar fashion in Alaska, starting in 1900. By 1910, there were approximately 18 dredges in use, with this total jumping to 42 by 1914.[3]

Dredges used in both regions were mainly the New Zealand style bucket-line dredges. Their design was somewhat adapted to accommodate working in frozen ground, but the initial design remained the same for the 80 years they were in service.[4] The bucket-line dredges were large, land-locked floating machines suitable for digging up ponds, allowing them to work across an entire area to be mined. They comprised a continuous line of buckets, also called a digging ladder, which scraped the bottom and edges of a pond. The buckets transported mud, rock, and gravel to a screening area. In the screening area, larger, heavier material was separated out. After the material was contained, tailings—the waste rock from dredging—would be deposited from the back of the dredge into the pond. Dredging allowed miners to process vast amounts of gravel to extract as much gold as possible leftover from the Klondike Gold Rush in a short period of time. On average They could be operated for a low cost and were highly efficient often operating on a 24-hour-a-day schedule.[5]  According to Yukon placer mining expert Norman Ross, dredges operating in the Yukon until the 1960s were extremely efficient, processing between 1.0 and 1.8 million cubic yards (765,000 to 1.4 million m3) per season. He notes that the demise of dredges in the Yukon in 1966 was not a reflection of their productive capability but rather, the heavy labor involved in pre-thawing the ground so mining could be carried out.[6] 

[edit] Suction Dredges

Today, most small-scale placer gold mining carried out with dredges is done with suction dredges, also referred to as modern dredges. The suction dredge is used primarily in the recovery of gold from within stream sediments. This style of dredge is extremely popular with recreational miners because it is affordable and easy to operate. A suction dredge operation can usually be accomplished with one or two people.[7]

A suction dredge suctions material up through a hose from the bottom of the river. The power used in suctioning comes from the jet-power intake. After material is suctioned up to the surface, it usually gets processed through a floating sluice at the surface. Any waste material from the box is then usually discharged over the side of the dredge.[8] Large suction dredges are also used in some commercial gold mines and are more efficient at extracting gold than their predecessors the bucket-line dredges were.[9]

[edit] Process/How it Works/Types

Gold dredges perform four functions essential to a mining operation: digging, classifying materials, capturing gold, and disposing of waste material.

There are four common types of gold dredges used, each operating with an endless chain of buckets to grab and retrieve payload from a river or creek bottom.

[edit] Flume Sluice Dredge

This dredge features buckets that dump payload directly onto the head of long sluice that runs down the axis of the boat. This type of dredge can produce large amounts of payload at the front, and then run the payload through a sluice without interfering with operations. The biggest advantage of the flume sluice dredge is its ability to work well in shallow waters in narrow, rich wet paystreaks. The bucket size is small, ranging from one-and-a-half to three cubic feet (0.04 to 0.1 m3) with the capability of processing about 500 cubic yards (382 m3) of material daily.[10]

[edit] Screen and Flume Dredge

Buckets on a screen and flume dredgeare used to deliver and dispense payload into a revolving screen, separating out the larger gravel from smaller material. The excess larger-sized gravel is then discharged off the stern through a chute. Another version consists of a flat table-like screen called a shaker or shaker deck, used to break apart the material before passing it through a sluice. This type of dredge is suitable for working ground with larger-sized gravel and rocks, but is still limited to working in shallow ground.[11]

[edit] Combination Dredge

The combination dredge combines a revolving screen with a conveyor belt or mechanical stacker to remove tailings and waste material astern of the boat. Smaller-sized payload passes through the revolving screen onto a wide riffled sluice that discharges the material onto sluices connected by two long flumes on either side of the hull.[12]

[edit] Table Stacker Dredge

This is the most familiar bucket-line gold dredge to operate in the gold fields. In a table stacker dredge, the line of buckets delivers payload to a hopper near the head of revolving screen. As it passes, small material is lost through a series of holes. The holes are smaller at the head of the screen and larger near the tail end. Larger material is carried along a conveyor and discharged off the stern in a similar fashion to a combination dredge. The smaller material falls down through a series of sluices with riffles, called tables. It continues to fall downwards into additional sluices until the finer gravel and sand is washed off the stern of the dredge through short chutes. It is the stacker, however, that removes most of the unwanted bulk material.[13]

[edit] References

  1. Dredging. New Zealand Government. 2008-12-02.
  2. Dredging. New Zealand Government.2008-12-02.
  3. Gold Dredges of the Yukon and Alaska. Quest Connect. 2008-12-02.
  4. Bucket-line Dredge. Prospectors Paradise. 2008-12-02.
  5. Gold Dredges of the Yukon and Alaska. Quest Connect. 2008-12-02.
  6. Placer Mining in the Klondike - Third Century. Norman Ross. Presentation.
  7. Suction Dredging for Gold. Dave McCracken. Gold and Treasure Hunter Magazine. Mar/Apr 1993. 2008-12-02.
  8. Bucket-Line Dredge. Prospectors Paradise. 2008-12-02.
  9. Gold Mining: Gold Miner Dredging. Great Western Mining. 2008-12-02.
  10. Gold Dredging. E-gold Prospecting. 2008-12-02.
  11. Gold Dredging. E-gold Prospecting. 2008-12-02.
  12. Gold Dredging. E-gold Prospecting. 2008-12-02.
  13. Gold Dredging. E-gold Prospecting. 2008-12-02.